Question/observation shooting the Vulcan .25: accuracy with/without moderator

[fe7565]

Cleaned barrel Vulcan .25 Gen2, JSB 25.39gr, indoor range, unsorted/unweighted pellet, 5-shot groups.

Is it normal for the shroud/moderator to move POI by inches but still have the same accuracy?

First, an (obvious) observation that proper gun hold or a rifle-stand can make a difference. I mostly used a gun rest such as the photo below with varying success in the past when testing my Vulcans. Because the rifle is not sitting secured with claws/vice-grips, I suspected the one-piece metal gun rest maybe causing too much random vibration when firing. 



So I decided to go back to a more simpler solution and used a plastic block for front rest and a stuffed rice-bag for the rear. With the new setup I had to change my hold on the rifle too. Not sure of it was the different rifle rest or my rifle hold, but it worked (see photos below).

Now, the other thing I suspected is that the shroud/moderator causing some random inaccuracy (albeit I am pretty sure not clipping). So I took the shroud/moderator off and shot with the bare barrel. The groups were about the same with or without the shroud/moderator. But when I put the shroud/moderator back on the barrel the POI shifted about one inch left and one inch down at 50 yards. The grouping was still the same as before. So, is that normal? Apparently, the pellet's flight is effected by the baffles, passages, and by the "second" crown which is basically the end of the moderator. Should I have the pellet passage enlarged, or should I add some micro-holes by the rear of the shroud to vent out the air in that direction and not towards the front? 


Shot WITHOUT the shroud/moderator, only naked barrel at 50 yards:


Shot WITH the shroud/moderator at 50 yards:


Shot WITH shroud/moderator at 75 yards:

 

[Guest]

It's normal for the POI to change anytime you add anything to the barrel, and hopefully keep the accuracy.

 

[fe7565]

So why does the POI change? And should it? Does the POI change because the shroud/moderator act as an extension to the barrel and the pellet is able to spin at a longer "virtual" twist rate? Or the POI changes because the baffles and end-cap of the moderator/shroud act as another crown?

 

[Guest]

It changes the harmonics , vibrations of the barrel. As long as it's still accurate is all that matters. Same thing happens in my rifle and probably all rifles. There was a thread on here a while back discussing this issue. When I remove my moderator my POI moves 3 inches right at 65 yards.

 

[FearnLoading]

My 75 yard shots are slightly more spread out than yours. Using either the 25 or 34 grains, my groups are a bit larger. Long ranges I can still hit a 3 inch target 8 out of 9 times but at 75 yards I am finding it tough to get the gun to give me smaller groups. I still need to get used to the gun I guess and I plan on cleaning my barrel this week. Hopefully that will reduce my group sizes. Will post results on this thread during the weekend. 

 

[fe7565]

@FastEddie....I will keep in mind when doing my initial scope zeroing at 30 feet in my house that a moderator can move the POI 3 inches right...it may save a window

@FernLoading....my barrel gets quite dirty even using JSBs and will be cleaning it every 100-150 shots while still testing. May be I need to check if it has any small burrs inside. But happy with the groups albeit I did not have enough time shooting with the new hold/rest to determine if the rifle can do this consistently at 75 yards or longer. I also need to weigh and sort pellets. For now also hand-feeding single pellets while testing in order to reduce any variables. 

Just bought a used Casio high-speed on Ebay ($90) and waiting for the camera to scope mount ($10) from China http://www.ebay.com/itm/111939078150?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT that can also be adapted for use with my iPhone. With that I will be able to confirm steady pellet flight.

The Vulcan is certainly very accurate, seems to be more accurate so far than what I am capable of shooting. So, it's a work in progress. Again, I am sure it's not new for many others but do not underestimate the rifle hold or rifle-rest setup. When I switched to a simple two piece bag setup, the groups became dramatically more consistent and accurate.

I think I need to work on my eye placement and cheek weld. I want to drop the cheek weld because it's hard to for me to place it on the bullpup in the same spot consistently. I think the cam or the iPhone will eliminate that problem.

 

[linsfreak]

Vulcan rocks!!!!!!

 

[PCP507]

The vulcan is a very accurate gun, but it´s definetely hold sensitive. In my experience with my .25 Vulcan it makes a huge difference the way you hold it, mine likes a light grip. She is delicate like that

 

[cactusjack]

I just bought a Vulcan 25 and had clipping issues with JSB 25.39 pellets. At 25 yds the group was about two inches but when I went out to 50 yds a few of them missed the entire target frame. I dismantled the moderator and found that the last three baffles were out of round. I do not think they were manufactured that way but resulted from shooting. The last one being obviously out of round. After talking to the dealer he mentioned that I should sand the opening of the baffles to expand the opening. I did this on the last three baffles and the problem was gone. At 25 yds it is a one hole group and about one half and inch at 50 yds. The rifle now performs nicely. There seems to be no noticeable increase in loudness.

 

[FunGun]

I have a vulcan .25 with an added moderator. I too noticed a shift in the POI of about an inch at 35 yards. Now that the gun is broke in, I'm not sure if the issue still exists since I have not taken the screw on moderator off. If I get time this weekend I will go shoot it without the moderator and with and see if the issue still exists or not.

 

[Mr.Gadgets]

"fe7565" Does the POI change because the shroud/moderator act as an extension to the barrel and the pellet is able to spin at a longer "virtual" twist rate? Or the POI changes because the baffles and end-cap of the moderator/shroud act as another crown?

The hole diameter of the moderator, and baffles, are slightly larger than the pellet. Therefore pellets do not touch the moderator, or baffles inside it at all. If there is a misalignment, then the pellet may "clip" the moderator on its way out.

 

[FunGun]

“fe7565″ Does the POI change because the shroud/moderator act as an extension to the barrel and the pellet is able to spin at a longer “virtual” twist rate? Or the POI changes because the baffles and end-cap of the moderator/shroud act as another crown?

Mr. Gadgets says: "The hole diameter of the moderator, and baffles, are slightly larger than the pellet. Therefore pellets do not touch the moderator, or baffles inside it at all. If there is a misalignment, then the pellet may “clip” the moderator on its way out."

Very good point and I agree about the "clipping" but I must also give some consideration to what fe7565 brings to point. 
"virtual twist" If the baffles are oversized but affect the airflow around the pellet (compresses the air) as the pellet approaches and passes thru the baffle I could see how this might have an effect on the POI due to changes in the airflow around the pellet. Air flow and turbulence of air is a science all into itself. Just look at the research that goes into aircraft wing design, boat hull design and even bicyclist. Interesting stuff, anyone got access to a wind tunnel we can play with? lol.

 

[fe7565]

Albeit, seems normal, there must be something going on if the POI changes that much after putting on a moderator and do not see any damage to the baffles from by the pellet. Just like a crown...the pellet never touches it but if the crown is messed up the pellets may go all over. So if the baffles are too close to the pellet's path, I would assume the baffles can act like a crown. What I read is that enlarging the pellet's path through the baffles/moderator will not affect the sound or anything else (i.e grouping) as long as it's within limits. What those limits are...I do not know. Neil, or one of the mod guys may have some golden rule how much you can go up in size in a given caliber.

I personally do not like that my POI moves by an inch or more when putting on a mod or shroud. Does it happen because the mod/shroud acts as an extension to the barrel and the pellet gets some extra twists as a result of the longer "barrel" which causes the pellet to spiral....thus the POI change? Or some other reason? Would rather opt for a larger pellet passage even if it leads to a marginally larger sound as long as there is no overall POI change.